Selective rearrangements of 1λ4,2,4-benzothiadiazines

Abstract

1-Aryl-1λ⁴,2,4-benzothiadiazines (1) rearrange when heated above 180 °C to give the isomeric 4-aryl-4H-1,2,4-benzothiadiazines (2) by an intramolecular pathway, a [1,4]-sigmatrophic shift. The 1-phenyl-1λ⁴,2,4-benzoselenadiazine (4) rearranges in a similar way, but 1-alkyl- and 1-morpholino-1λ⁴,2,4-benzothiadiazines (6) and (8) give 2H-1,2,4-benzothiadiazines (7) with loss of the 1-substituent. On irradiation, the 1-aryl- and 1 -alkyl-benzothiadiazines (1), (6), and (13) rearrange to give benzimidazoles by cleavage of the sulphur^–nitrogen ylide bond; an intermediate nitrene can be intercepted by dimethyl sulphoxide.